Optical fibers are useful as communication links, in order to control flying objects. One example of such a controlled flying object is, e.g., an unmanned aircraft. The flying object normally contains an optical fiber canister, on which several kilometers of optical fiber are wound, which unwind during the flight of the object. The free end of the fiber is connected to the launching position of the object, so that physical connection exists and either mono- or bi-directional communication is possible through the fiber between the flying object and its launching position.
Optical fibers are filaments made of optically pure glass processed so that they are capable of transmitting light therethrough with high efficiency, thus transmitting a large fraction of the light which is directed into the fiber even for long distances, such as many kilometers. Glass fibers are excellent for transmitting communication, and have the considerable advantage of being light in weight and small in diameter. The optical fiber is normally made of silicon dioxide and is coated with a so-called "buffer", which is a polymer layer which is provided to protect the optical fiber from scratches and other surface damages.
While reference is made throughout this specification to "flying objects", its is understood that the same considerations apply also, e.g., to objects moving through water, mutatis mutandis, and that the invention is also directed to objects capable of moving through any fluid, such as underwater remotely operated vehicles and the like. Reference to air as the representative fluid is made only for the sake of brevity.
One basic problem which has to be dealt with is to provide a flying object incorporating an optical fiber which is constructed so as to withstand the tensional loads applied to it during flight, and to avoid breakage of the fiber which results in a substantial loss of the object due to loss of communication. It should be noted that the problem of providing a correctly dimensioned fiber is not a minor one. The preferred application is based on winding the bare fiber itself, e.g., a silica core, typically 125 microns in diameter, coated with a thin buffer layer, typically 20-70 microns thick. In this way the fiber core is used as both a lightguide and a strength member, and the tether cable weight, volume and cost are reduced to a minimum. nevertheless, other applications that include an additional strength member (like reinforcement fibers) are also possible.
Because of a statistical distribution of the silica strength, a tensile proof test is used to assure a minimum tensile strength along the whole fiber length, which is supposed to assure that it will withstand the payout loads. Application of high proof test loads to the fiber results in higher failure density during the test (failure density=number of failures per unit length).
At high failure densities the probability of obtaining the desired fiber length in one piece becomes low, which means that either the yield of the process becomes low, or many splices need to be performed to achieve the desired length. In either way, the result is an increase in the fiber cost. Therefore, too weak a fiber will result in rupture, but excessively strong a fiber will be very expensive in the case of a bare fiber, or will add unnecessary weight and volume to the flying object, in the case of a reinforced fiber, thus resulting in shortened flying ranges. As will be appreciated by the skilled person, since the optical fiber employed is several kilometers long, any addition of weight or cost required to improve fiber strength has a substantial influence on the cost and performance of the final object.